Blue Door

From Wikitroid, the Metroid wiki

Blue Doors are doors that can be opened by any weapon. They are found in every Metroid game except Metroid II: Return of Samus and Metroid Fusion (the latter is replaced by a Blue Hatch). The blue color is actually the color of a force field covering the door. After shooting a blue colored door, the energy covering it will disrupt and peel off or dissipate, depending on the model. The main purpose of the energy covering the blue doors is to avoid accidental/unintentional entries to rooms by wild creatures surrounding the area. This blocks off a large quantity of bioforms from access since most of them can't manipulate weapons or simply don't have the means to produce an energy-based projectile.

In Metroid Prime 3: Corruption, by scanning a blue door on Planet Elysia (once inhabited by the Chozo), it is revealed that since the discovery of said planet, the Galactic Federation used the model of the Elysian blue door as a basis to all currently existing doors found throughout the galaxy on planets, ships, and stations under Federation control.

[edit] Gallery

Blue doors often vary in size shape and appearance, as demonstrated below.

A Tallon IV Blue Door.	An Aetherian Blue Door.	A GF Door.	A Bryyonian Blue Door.
An Elysian Blue Door.	A Space Pirate Blue Door.